1. Field of the Invention
This invention relates to electronic circuits used to generate a power-on reset signal and more specifically to an electronic circuit used to generate a power-on reset signal with adjustable hysteresis.
2. Description of the Relevant Art
The problem addressed by this invention is encountered by systems which depend on a good voltage source for the reliable operation of the system. Common examples of such systems are a microprocessor based system such as a personal computer, an automobile, or a radio. In fact, most consumer and non-consumer products which use electronics have a way to enable the operation of the electronics, such as a power-on reset circuit, when a reliable power source is available and disabling the operation of the electronics when the reliable power source is not available.
In a personal computer, a power supply converts alternating current voltage into a direct current voltage, typically 5 volts. When the computer is turned on, the power supply may take hundreds of milliseconds to reach a stabilized voltage of around 5 volts. However, the microprocessor in the personal computer can operate unreliably at voltages below 4.5 volts. Since the microprocessor is capable of operating thousands of instructions in a single millisecond and can run unreliably at voltages below 4.5 volts, running the microprocessor at voltages below 4.5 volts could yield disastrous results. Data could be corrupted or lost. Erroneous messages or command signals could be sent. The possibilities for disaster are endless. Consequently, system designers typically disable the microprocessor until the power supply has reached a sufficient and stable voltage for the microprocessor to operate reliably. By disabling the microprocessor with a power-on reset circuit until the power supply provides a known good voltage, errors in the microprocessor due to low voltage and/or noise are avoided.
FIG. 1 shows a typical power-on reset circuit 12 as known in the prior art. In this circuit, Vcc voltage 2 represents the voltage from a power supply which is not shown and POR 10 represents the power-on reset signal which is generated by a Schmitt trigger 8. Before the power supply is turned on, the POR 10 is low since capacitor 6 is discharged and since the Schmitt trigger 8 does not have any source of power (since it is also powered by the power supply). When the power supply is turned on, the Vcc voltage 2 rises and begins to charge capacitor 6 through resistor 4 and power the Schmitt trigger 8. Until the Schmitt trigger 8 has sufficient voltage to operate, its output is in an indeterminate state. When the voltage on capacitor 6 reaches the threshold voltage of the Schmitt trigger 8, the output of the Schmitt trigger changes to a high state and thus the POR signal 10 swings to a high state.
Prior art power-on reset circuit 12 in FIG. 1 is limited to situations where Vcc has a rise time at turn-on that is much faster than the RC time constant. As a result, large resistors and capacitors are required for those power supplies which have long rise times. The large capacitors can use a significant amount of area on an integrated circuit. Additionally, the POR output is indeterminate at VCC voltages below the operating voltage of the Schmitt trigger 8.
FIG. 2 shows a second power-on reset circuit 28 as known in the prior art. In this circuit, Vcc voltage 14 represents the voltage from a power supply and POR signal 27 represents the power-on reset signal which is generated by the comparator 26. In operation, POR signal 27 is initially at a low voltage as Vcc voltage begins to rise from a power supply off condition. The voltage at the non-inverted input of comparator 26 is controlled by the voltage divider created by resistor 22 and resistor 24. The voltage at the inverted input of comparator 26 is controlled by the voltage divider created by resistor 16 and diodes 18 and 20. As Vcc 14 rises, the non-inverted input rises proportional to Vcc 14 as defined by the resistance of resistor 22 and 24. The inverted input rises in voltage with the Vcc voltage 14 until Vcc 14 reaches the threshold voltage of diodes 18 and 20, typically 0.7 volts each. POR 27 goes high when: EQU (Vcc *r.sub.24)/r.sub.22 +r.sub.24 &gt;V.sub.18 +V.sub.20
Where
Vcc is the voltage of Vcc 14, PA1 r.sub.22 is the resistance of resistor 22, PA1 r.sub.24 is the resistance of resistor 24, PA1 V.sub.18 is the forward voltage drop of diode 18, PA1 V.sub.20 is the forward voltage drop of diode 20.
The problem with this prior art circuit is that signal POR 27 is indeterminate when the Vcc voltage 14 is below the operating voltage of the comparator 26. Additionally, the circuit does not have any hysteresis or is limited to the hysteresis of comparator 26 which is not easily modified. Additionally, the component count is high when the components which are required to make a comparator are included.